There exist many applications in industry and science in which it is required to determine the number of particles suspended in a fluent (i.e., liquid or gaseous) medium, and the sizes or size distribution of such particles. Such measurements can be very important in manufacturing processes in a number of industries, including pharmaceuticals, plastics, chemicals, and paper. Processes such as crystal growth, precipitation, polymerization, gravimetric separation and grinding must be monitored with regard to the sizes of suspended particles, to control the quality of the final product. Ideally, particle measurements should be made on-line, to provide real time information for process control, and to avoid distorting particle size information by removing samples from the process.
In the past, a number of different technologies have been developed for the analysis of particle size. One prior optical technique, described in U. S. Pat. No. 4,871,251, utilizes a focused light beam from a laser diode source. Using this technique, one is able to obtain a very small sensing volume at a focal spot in the fluent medium, and therefore a very high light intensity at the focal spot. This intensity is high enough to overcome the natural falloff in the intensity of light scattered into backward angles. This makes individual light pulses, coming from individual particles going through the focal spot, detectable by ordinary detection means. This prior technique also includes scanning the focused light beam at a near-constant speed, such that by measuring the pulse length of each of the light pulses, one can determine particle size. One can implement mechanical scanning mechanisms relatively easily for scanning rates up to about 3 meters per second.
The technology described above can in principle be used for airborne particles. Such particles are encountered in dry milling and grinding applications, and in applications requiring the measuring of the size of vapor droplets such as those coming out of injector nozzles in fuel injection systems or in spray nozzles used in the fertilizer industry. In practice, however, the typical flow velocities for airborne particles are much higher than the scanning rates readily obtainable from mechanical scanning mechanisms. For example, in fluidized flows, powders are air-conveyed at velocities as high as 30 meters per second. In order to make an optical particle measurement system insensitive to flow velocity, the laser beam would have to be scanned at a velocity much higher than the material flow. While one can design higher speed electronic detection circuits, it is generally not feasible to implement mechanical scanning means that can achieve the required scanning rates.
One possible solution for measuring airborne particles would be to introduce baffles into the material flow, in an attempt to slow down the flow rate. It has been found, however, that this approach leads to agglomeration effects, causing particles to stick together, therefore producing an erroneous reading. In practice, it has been found to be very difficult to get a well dispersed dry powder sample. Another possible approach would be to use off-line measurements. However such measurements are cumbersome, time-consuming, and cannot be done in real time.
As a result of these consideration, there is a strong, presently unmet need for an effective means for measuring particle size for airborne powders and the like. Some attempts have been made to make measurements at very low material concentrations, where the laser beam can pass through the material, and where only a few particles are illuminated at any one time. In such a case, one can use the traditional Frauenhofer diffraction measurements. This method, however, fails at the higher material concentrations that are typically encountered in most applications.